1. Field of the Invention
The present invention relates to a digital interface system, and more particularly to a multi-channel multiplexing switching system and an inter-office signal system in a digital switching network arranged by connecting time-division switching systems through digital lines.
2. Description of the Prior Art
A time division switching system handles calls such as voice and data, having various bit rate. A circuit switching system handling 64 kbps as a unit can not perform multi-channel switching as achieved in a packet switching system. Accordingly, in order to increase transmission efficiency, generally, a time slot on a transmission line is fixedly assigned previously for each kind of bit rate in accordance with an attribute of a terminal.
On the other hand, JP-A-1-120931 discloses a system in which a link between opposite offices is established in accordance with generation characteristics of calls. This system has an improved transmission efficiency as compared with a system having the fixedly assigned time slots but does not include means for informing the opposite office (opposite switch) of attribute information of a multi-channel call.
Further, JP-A-1-291554 discloses a system in which sub-rate call information is transferred by the inband signalling channel in the sub-rate switching using multiplex circuit.
In the above described conventional systems, since the conventional system disclosed in JP-A-1-120931 does not include means for informing the opposite office (opposite switch) of the attribute information of a multi-channel call, it is necessary to handle multi-channel calls having coincident recognition between opposite offices and accordingly there is a problem in the effective utilization of circuits.
Further, as disclosed in JP-A-1-291554, the system for transferring the sub-rate call information by the inband signalling channel in the sub-rate switching using multiplex circuit has the following problems.
(1) Two-stage call setting procedures of a procedure for setting a B channel call of 64 kbps and a procedure of setting a sub-rate call on the B channel are required, and the call control structure is complicated.
(2) In transfer of sub-rate call setting information by the inband, it is difficult to transfer facility information for permitting additional services between switching systems.